Electronics specs questions

Jim Pieronek jvp%fuelrod at juliet.ll.mit.edu
Fri Nov 4 21:17:38 GMT 1994


MTN-KAT writes:
 > I don't know diddly about electronics. I'm sitting here looking thru a
 > Digikey catalog trying to figure out which mosfet power transistors
 > and which opto-isolators to use for my project.  the mosfets list
 > BVdss, Ip, Idm, Vgs, Rds and PD. From my reading in Practical
 > transistors and Linear IC's book, BVdss is the voltage to trigger the
 > transistor. All of these mosfets are 60V-120V, obviously I'm only
 > running 13.5V max, what am I missing?
 > 

BVdss is the breakdown voltage, drain to source, with the gate
connected to the source (N-channel enhancement-mode FET off state).
It is the voltage at which the transistor blows up, not where it turns
on.  Vgs(on) or Vgs(th) is the turn-on threshold voltage - usually 2.0
to 4.0 volts.  BVgs is the max. gate voltage.

 > The opto-isolators list BVCEO(min), the smallest is a 7V with a
 > current transfer ratio of 250%, no specs for on/off time. From my
 > reading a darlington output should boost the current, the catalog
 > doesn't say how much.  The isolation voltage is only 3000V for this
 > particular item, some list 5300V would they be better for isolating
 > the engine noise from the electronics?
 > 

BVceo(min) is the minimum breakdown voltage from the collector to the
emitter with base open.  Another blow-up parameter.  Current transfer
ratio tells you what the current boost is - put 1mA into the LED on
that optoisolator and it will try to pass 2.5mA through the
transistor.  Isolation voltage should be more than enough.

 > I had ordered some logic level opto-isolators and they came with
 > schmitt trigger outputs, I tried to run these at slow speed 0.1 sec
 > and they just stayed in on mode, the data rate is supposed to be
 > 1.0MHz. Could I have been hooking them up wrong?
 > 

Yes, this is actually possible, particularly in cases where they don't
work.

 > Is there a cross-reference manual available, I have a listing for some
 > logic level IGBT's, HGTP15N40C1 but Digikey doesn't have any part
 > numbers similar to this. The MOSFETs have a part number, IRF540, once
 > again no similar numbers in the catalog.
 > 

IRF540-ND is in the Digi-Key catalog.  Look on the page with
International Rectifier HEXFETs.




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